The Z-space transponder is a piece of Andalite technology that can communicate at faster than light speeds through Zero-Space. Functionally, the Animorphs and Ax use it to call the Andalite homeworld and to intercept Yeerk transmissions for intelligence purposes. The Yeerks will sometimes use transponder-modified devices to communicate in places where normal communication is hampered - for instance, to use a cell phone far underground.

Later on, in The Revelation, the Z-Space transponder is convergently invented by humans, specifically Marco's dad and his engineer co-workers. Marco describes it as "mini-cooler size", but by The Sacrifice, it was small and light enough for Ax to carry as a raccoon and horned-owl.

The Predator - Ax needs a Z-Space Transponder to complete a distress beacon he is making so he can summon and then steal a Bug fighter, in order to return to the Andalite homeworld to inform them about the invasion of earth. They go to Chapman's basement and successfully steal a transponder, but are thwarted in their attempt to steal a Bug fighter after calling for it.

The Revelation - Marco's dad, an engineer, invents a Z-space transponder. When he tells Marco and Nora this at dinner, Marco is immediately worried that his dad is next on the list to become a Controller. He tells the rest of the Animorphs, and later that night his Dad gets a call that one of his co-workers has died in a car crash, and he is needed to comfort his colleague's wife. He leaves immediately, and Marco, knowing it's a trap, follows him. Marco rescues his dad from infestation and explains everything about the Yeerks and the war to him. Once his dad is safely given shelter at Mr. King's house, Ax and the Animorphs reason that if his dad was able to invent a Z-space transponder using supplies from earth, he should be able to recreate it, and the Animorphs can use it to intercept Yeerk transmissions or call the Andalite homeworld. After some time working, Marco's dad is able to recreate it with Ax's help. Once they finish a rudimentary version of it, which is the size of a mini-cooler, they start to receive Yeerk communications, some of which Ax can interpret. One thing they find out is that Visser One has returned to earth and is to be executed as a traitor. Ax also can understand that a new tunnel to the Yeerk pool has been added, which leads to a facility to dock and repair Bug fighters. So, Marco pretends to be a hunter who has come across a Hork-Bajir, and calls the police telling them that. Very soon, a bug fighter arrives to the spot he is calling at. The Animorphs are then able to steal that bug fighter to go to the Yeerk Pool and carry out their mission. Later, Ax uses the Z-space transponder to call the Andalite homeworld.

The Deception - During their call to the Andalite homeworld, they are able to briefly talk to an Andalite commander and warn the Andalites about the Anati situation. However, they are quickly interrupted by Bug fighters. They grab the transponder and hurry away.

The Sacrifice - Ax, in raccoon morph, takes the Z-space transponder away from the Hork-Bajir valley where the Animorphs now live, and calls the Andalite homeworld. Bug fighters soon arrive after detecting the signals made by the transponder, and Ax leaves quickly in owl morph as soon as he is aware of this.

The Hork-Bajir Chronicles - Aldrea, who is on the Hork-Bajir homeworld, wishes for a Z-space transponder to contact the Andalite fleet or homeworld after her family is destroyed, because a radio signal would take decades to reach anyone.